For example, a circuit board that is arranged in an electronic device has a board body, electrode pads arranged on a surface of the board body, and a solder resist. The solder resist is arranged on the surface of the board body, has openings formed at positions corresponding to the electrode pads, and serves as a protective film. The solder resist has a function of preventing a solder material supplied onto the electrode pads from spreading onto the circuit board when the solder material is melted. The solder resist also has a function of protecting the surface of the circuit board.
In order to supply solder paste onto the electrode pads of the circuit board, a so-called printing method is used in some cases. In the printing method, while a solder mask (printing plate) is placed on the solder resist of the circuit board, the solder paste is supplied onto the electrode pads located in the openings of the solder resist through mask openings formed in the solder mask.
In order to mount an electronic part on the circuit board, lead terminals of the electronic part are placed on the solder paste supplied onto the electrode pads of the circuit board, and the solder paste is heated in a reflow furnace. When the solder paste is melted, the lead terminals of the electronic part, which are placed on the solder paste, sink into the solder paste owing to the weight of the electronic part. This causes the lead terminals of the electronic part to be bonded to the electrode pads of the circuit board by a solder material contained in the solder paste and to be electrically connected to the electrode pads.
A related-art document is a Japanese Laid-open Patent Publication No. 10-335798.
The heights of the lead terminals of the electronic part are not necessarily equal to each other. The circuit board or the electronic part is not necessarily formed in a shape according to design. In addition, deformations such as warpage may occur in the circuit board and the electronic part. Thus, a “variation” in interval between the electrode pads and the lead terminals normally occur.
When the amount of the solder paste supplied onto the electrode pads is small, and the electronic part sinks into the solder paste owing to the melt of the solder paste, the solder paste may not accommodate a “variation” in interval between the electrode pads and the lead terminals, and a connection between at least one of the lead terminals and the interested electrode pad may fail. Especially, in recent years, the sizes of electrode pads have tended to be reduced in conjunction with a reduction in a pitch of the electrode pads (and lead terminals). Thus, the amount of the solder paste to be supplied onto the electrode pads has been reduced. Therefore, the connections between the electrode pads and the lead terminals may easily fail.
In order to reduce the rate of the occurrence of a failure of the connections between the electrode pads and the lead terminals, the thickness of the solder mask is increased, for example, from 150 μm to 200 μm, and the amount of the solder paste to be supplied onto the electrode pads is increased in some cases. However, when the thickness of the solder mask is increased and the solder mask is removed from the circuit board, the solder paste may remain in the mask openings, and a sufficient amount of the solder paste cannot be supplied onto the electrode pads, or the sufficient amount of the solder paste cannot be supplied onto the electrode pads, and the solder paste cannot accommodate a variation in height of the lead terminals.
In order to increase the amount of the solder paste to be supplied onto the electrode pads without an increase in the thickness of the solder mask, it can be considered that the sizes of the openings that are formed in the solder resist at the positions corresponding to the electrode pads are increased, for example. However, the increases in the sizes of the openings that are formed in the solder resist are limited in consideration of the fact that the solder resist has the function of protecting the circuit board.
In addition, in order to increase the amount of the solder paste to be supplied onto the electrode pads without an increase in the thickness of the solder mask, it can be considered that the thickness of the solder resist is increased so that the depths of the openings that are formed in the solder resist at the positions corresponding to the electrode pads are increased, for example. However, when the thickness of the solder resist is increased, the thickness of the solder paste supplied onto the electrode pads is also increased. Thus, when the lead terminals of the electronic part sink into the solder paste, a large amount of the solder that exists in the openings may flow onto the circuit board, and a so-called solder bridge may be formed.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.